87Sr/86Sr isotope ratios in trees as an archaeological tracer: Limitations of linking plant-biomass and bedrock Sr isotope signatures

Abstract

Assuming that strontium (Sr) isotope ratios in trees mirror the bedrock isotope signal, between-site 87Sr/86Sr comparisons can serve as a powerful tool in archaeological wood provenance studies. Here we discuss the strengths and weaknesses of this approach using Sr isotope data for bedrock, trees and rainfall collected at seven sites in the Czech Republic. The study included basalt, durbachite, granite, paragneiss, limestone and phyllitic slate. The range of 87Sr/86Sr ratios of these bedrock types was extremely large (0.704–1.743). The investigated tree species included oak, pine, and spruce. Within-tree variability in 87Sr/86Sr ratios (~0.010) was two orders of magnitude smaller than the variability in Sr ratios across the studied bedrock types. Oak and pine growing on identical bedrock exhibited statistically indistinguishable 87Sr/86Sr ratios. In four types of spruce tissues from a particular site, the 87Sr/86Sr ratios were indistinguishable, while significantly different from spruce tissues at another site. The 87Sr/86Sr ratios of the wood differed from those of bedrock, converging to the 87Sr/86Sr ratio of rainfall (0.709–0.714). Even at a 450-km distance from the nearest sea, atmospheric Sr partly originated from sea-spray. Incorporation of atmospheric Sr in trees often results in overlapping 87Sr/86Sr ranges and hampers source identification of archaeological wood.